1. Field of the Invention
This invention relates to a magnetic component for an electric machine, such as a motor; and more particularly, to a low core loss, unitary amorphous metal component, such as a rotor or stator, for a high efficiency, axial-flux electric motor.
2. Description of the Prior Art
Rotating electric machines almost always comprise at least two magnetic components, a stationary component termed a stator and a rotor appointed to rotate relative to the stator and about a defined rotation axis. Such a rotating machine allows energy to be exchanged between electrical and mechanical forms. Most familiarly, an electric motor is provided with a source of electrical energy, as from a battery or the electric power grid, that may be converted to usable mechanical work. On the other hand, a generator takes imposed mechanical work and converts it to electrical energy that may be used to operate other devices. In many cases the same structure may be used for both functions, depending on how the machine is connected electrically and mechanically.
A vast majority of rotating electrical machines operate electromagnetically. In such machines the rotor and stator normally comprise ferromagnetic materials. The components are used to either produce or direct a pattern of magnetic flux that varies either temporally or spatially or both. The conversion of energy between electrical and mechanical forms occurs in accordance with the well known principles of electromagnetism, especially Faraday""s and Ampere""s laws. In electromagnetic machines at least one of the rotor and stator is constructed using a soft ferromagnetic material and provided with a winding appointed to carry an electrical current and generate a magnetic field. Depending on the type of motor, the other component includes either permanent (hard) magnetic material or soft magnetic material that is excited by current-carrying windings or by induction. The soft magnetic materials most commonly used are low carbon steels and silicon-containing electrical steels, both of which are crystalline metallic materials.
The stator and the rotor in a machine are separated by small gaps that are either (i) radial, i.e., generally perpendicular to the axis of rotation of the rotor, or (ii) axial, i.e., generally parallel to the rotation axis and separated by some distance. In an electromagnetic machine, lines of magnetic flux link the rotor and stator by traversing the gaps. Electromagnetic machines thus may be broadly classified as radial or axial flux designs, respectively. The corresponding terms radial gap and axial gap are also used in the motor art.
Radial flux machines are by far most common. The rotors and stators used in such motors are frequently constructed of a plurality of laminations of electrical steel that are punched or otherwise cut to identical shape, stacked in registry, and laminated to provide a component having a requisite shape and size and sufficient mechanical integrity to maintain the configuration during production and operation of the motor.
One common design for a stator is generally cylindrical and includes a plurality of stacked laminations of non-oriented electrical steel. Each lamination has the annular shape of a circular washer along with plural xe2x80x9cteethxe2x80x9d that form the poles of the stator. The teeth protrude from the inner diameter of the stacked laminations and point toward the open center of the cylindrical stator. Each of the laminations is typically formed by stamping mechanically soft, non-oriented electrical steel into the desired shape. The formed laminations are then stacked in registry and bonded to form a stator. During operation, the stator is periodically magnetized by a magnetic field, produced by a flow of electric current in windings that encircle the teeth of the stator. Such magnetization is needed to drive the motor; but causes unavoidable losses due to magnetic hysteresis. These losses contribute to an overall reduction in motor efficiency.
Axial flux designs are much less commonly used, in part because of the lack of suitable means for constructing components having the requisite electromagnetic properties and adequate mechanical integrity. Certain disclosures have suggested axial flux motor designs, including those found in U.S. Pat. No. 4,394,597 to Mas and U.S. Pat. No. 5,731,649 to Caamano. These teachings also suggest magnetic components that employ amorphous metals.
Although amorphous metals offer superior magnetic performance, including reduced hysteresis losses, when compared to non-oriented electrical steels, they have widely been regarded as not suitable for use in electric motors due to certain physical properties and the resulting impediments to conventional fabrication. For example, amorphous metals are thinner and harder than non-oriented steel. Consequently, conventional cutting and stamping processes cause fabrication tools and dies to wear more rapidly. The resulting increase in the tooling and manufacturing costs makes fabricating amorphous metal components such as rotors and stators using conventional techniques commercially impractical. The thinness of amorphous metals also translates into an increase in the number of laminations required for a component of a given stack height, further increasing its total manufacturing cost.
Amorphous metal is typically supplied in a thin continuous ribbon having a uniform ribbon width. However, amorphous metal is a very hard material, making it very difficult to cut or form easily. Once annealed to achieve peak magnetic properties, it becomes very brittle, making it difficult and expensive to use conventional approaches to construct amorphous metal magnetic components. The brittleness of amorphous metal also causes concern for the durability of a motor or generator that utilizes amorphous metal magnetic components. Magnetic stators are subject to extremely high magnetic forces, which change at very high frequencies. These magnetic forces are capable of placing considerable stresses on the stator material, and may damage an amorphous metal magnetic stator. Rotors are further subjected to mechanical forces due both to normal rotation and to rotational acceleration when the machine is energized or de-energized and when the loading changes, perhaps abruptly.
Another problem with amorphous metal magnetic components is that the magnetic permeability of amorphous metal material is reduced when it is subjected to physical stresses. This reduction in permeability may be considerable depending upon the intensity of the stresses on the amorphous metal material, as indicated by U.S. Pat No. 5,731,649. As an amorphous metal magnetic stator is subjected to stresses, the efficiency at which it directs or focuses magnetic flux is reduced, resulting in higher magnetic losses, reduced efficiency, increased heat production, and reduced power. This phenomenon is referred to as magnetostriction and may be caused by stresses resulting from magnetic forces during the operation of the motor or generator, mechanical stresses resulting from mechanical clamping or otherwise bonding or fixing the magnetic stator in place, or internal stresses caused by the thermal expansion and/or expansion due to magnetic saturation of the amorphous metal material.
A limited number of non-conventional approaches have been proposed for constructing amorphous metal components. For example, U.S. Pat. No. 4,197,146 to Frischmann discloses a stator fabricated from molded and compacted amorphous metal flake. Although this method permits formation of complex stator shapes, the structure contains numerous air gaps between the discrete flake particles of amorphous metal. Such a structure greatly increases the reluctance of the magnetic circuit and thus the electric current required to operate the motor.
In order to avoid stress-induced degradation of magnetic properties, U.S. Pat. No. 5,731,649 discloses constructing amorphous metal motor components using a plurality of stacked or coiled sections of amorphous metal, and mounting these sections in a dielectric enclosure. The ""649 patent further discloses that forming amorphous metal cores by rolling amorphous metal into a coil with lamination, using an epoxy, detrimentally restricts the thermal and magnetic saturation expansion of the coil of material, resulting in high internal stresses and magnetostriction that reduces the efficiency of a motor or generator incorporating such a core.
The approach taught by German Patents DE 28 05 435 and DE 28 05 438 divides the stator into wound pieces and pole pieces. A non-magnetic material is inserted into the joints between the wound pieces and pole pieces, increasing the effective gap, and thus increasing the reluctance of the magnetic circuit and the electric current required to operate the motor. The layers of material that comprise the pole pieces are oriented with their planes perpendicular to the planes of the layers in the wound back iron pieces. This configuration further increases the reluctance of the stator, because contiguous layers of the wound pieces and of the pole pieces meet only at points, not along full line segments, at the joints between their respective faces. In addition, this approach teaches that the laminations in the wound pieces are attached to one another by welding. The use of heat intensive processes, such as welding, to attach amorphous metal laminations will recrystallize the amorphous metal at and around the joint. Even small sections of recrystallized amorphous metal will normally increase the magnetic losses in the stator to an unacceptable level.
Moreover, amorphous metals have far lower anisotropy energies than other conventional soft magnetic materials, including common electrical steels. As a result, stress levels that would not have a deleterious effect on the magnetic properties of these conventional metals have a severe impact on magnetic properties important for motor components, e.g. permeability and core loss. For these reasons, U.S. Pat. No. 5,731,649 discloses a magnetic component comprising a plurality of segments of amorphous metal carefully mounted or contained in a dielectric enclosure without the use of adhesive bonding.
Notwithstanding the advances represented by the above disclosures, there remains a need in the art for improved amorphous metal motor components that exhibit a combination of excellent magnetic and physical properties needed for high speed, high efficiency electric machines, especially axial flux designs. Construction methods are also sought that use amorphous metal efficiently and can be implemented for high volume production of axial flux motors and the components used therein.
The present invention provides a single-piece or unitary amorphous metal magnetic component for a high efficiency, axial-flux electric motor. The component may be a rotor or stator. In one embodiment the component comprises a cylinder of annular cross section having cylindrical inner and outer surfaces, two opposed annular faces, an axial thickness separating the faces and a plurality of radial slots in at least one of the faces for receiving electrical windings. The cylinder is formed of spirally wound amorphous metal strip. The layers are preferably electrically insulated from one another to reduce eddy current losses. The unitary construction eliminates all magnetic gaps within the component, thereby providing a closed path through which magnetic flux may flow. The term xe2x80x9celectric motor,xe2x80x9d as used herein, refers generically to a variety of rotating, dynamoelectric machines which, in addition to ordinary electric motors, may include electric generators as well as regenerative motors that may be operated optionally as electric generators.
The present invention further provides a bulk amorphous metal magnetic motor component which exhibits very low core loss under periodic excitation. As a result, the magnetic component is operable at frequencies ranging from DC to as much as 20,000 Hz. It exhibits improved performance characteristics when compared to conventional silicon-steel magnetic components operated over the same frequency range. The component""s operability at high frequency allows it to be used in fabricating motors that operate at higher speeds and with higher efficiencies than possible using conventional components. A magnetic component constructed in accordance with the present invention and excited at an excitation frequency xe2x80x9cfxe2x80x9d to a peak induction level xe2x80x9cBmaxxe2x80x9d may have a core loss at room temperature less than xe2x80x9cLxe2x80x9d wherein L is given by the formula L=0.0074 f (Bmax)1.3+0.000282 f1.5 (Bmax)2.4, the core loss, the excitation frequency and the peak induction level being measured in watts per kilogram, hertz, and teslas, respectively. The magnetic component may have (i) a core-loss equal to or less than about 1 watt-per-kilogram of amorphous metal material when operated at a frequency of approximately 60 Hz and at a flux density of approximately 1.4 Tesla (T); (ii) a core-loss equal to or less than about 12 watts-per-kilogram of amorphous metal material when operated at a frequency of approximately 1000 Hz and at a flux density of approximately 1.0 T; or (iii) a core-loss equal to or less than about 70 watt-per-kilogram of amorphous metal material when operated at a frequency of approximately 20,000 Hz and at a flux density of approximately 0.30T.
The unitary amorphous metal magnetic component of the present invention can be manufactured using numerous ferromagnetic amorphous metal alloys. Generally stated, the amorphous metal consists essentially of an alloy having the formula: M70-85 Y5-20 Z0-20, subscripts in atom percent, where xe2x80x9cMxe2x80x9d is at least one of Fe, Ni and Co, xe2x80x9cYxe2x80x9d is at least one of B, C and P, and xe2x80x9cZxe2x80x9d is at least one of Si, Al and Ge; with the proviso that (i) up to ten (10) atom percent of component xe2x80x9cMxe2x80x9d can be replaced with at least one of the metallic species Ti, V, Cr, Mn, Cu, Zr, Nb, Mo, Ta, Hf, Ag, Au, Pd, Pt, and W, (ii) up to ten (10) atom percent of components (Y+Z) can be replaced by at least one of the non-metallic species In, Sn, Sb and Pb, and (iii) up to about one (1) atom percent of the components (M+Y+Z) can be incidental impurities.
The present invention also provides a method for constructing a low core loss, unitary amorphous metal motor component. Generally stated, the method comprises the steps of: (i) spirally winding ferromagnetic amorphous metal strip or ribbon material to form a wound cylinder of annular cross-section having cylindrical inner and outer surfaces and two annular faces, the faces being separated by an axial thickness; (ii) heat treating the cylinder; (iii) adhesively bonding each of the layers of the wound cylinder to the layers adjacent thereto with an adhesive; and (iv) forming the component by cutting a plurality of slots in at least one of the annular faces, the slots extending generally radially between the inner surface and the outer surface and having a depth less than the axial thickness. Preferably the adhesive bonding is carried out by impregnation. Optionally, a finishing step (v) is carried out comprising coating the component with a suitable surface finish. The heat-treating step comprises one or more heat treatments to modify the mechanical or magnetic properties of the amorphous metal feedstock. Such optional heat treatments facilitate machining operations and improve the magnetic properties of the component. Steps (i) to (v) may be carried out in a variety of orders and using a variety of techniques including those set forth hereinbelow.
The present invention is also directed to a unitary amorphous metal component constructed in accordance with the above-described methods. In particular, a unitary amorphous metal magnetic motor component constructed in accordance with the present invention exhibits low core loss and is suited for use as a stator in a high efficiency, axial flux, electric machine.
The present invention further provides an axial flux dynamoelectric machine, including a motor, generator, or regenerative motor incorporating the above-described unitary amorphous metal magnetic component. In an aspect of the invention, the motor is of the induction type and incorporates at least one unitary amorphous metal stator component. The induction motor optionally also incorporates a unitary amorphous metal rotor. In another aspect, the motor is a brushless, axial-flux, permanent magnet DC motor having a generally cylindrical, unitary amorphous metal stator that comprises a plurality of tooth-shaped pole sections protruding axially from the generally annular back iron region and integral therewith. The motor also comprises a disk-like rotor having at least one permanently magnetized section with at least one pair of oppositely directed magnetic poles and bearing means for rotatably supporting the stator and rotor in a predetermined position relative to each other. The magnetic poles of the rotor are located on the disk surface and generate magnetic flux directed generally perpendicular thereto.
The advantages afforded by the present invention include simplified manufacturing, reduced manufacturing time, decreased stresses (e.g., magnetostrictive) encountered during construction of bulk amorphous metal components, and optimized performance of the finished amorphous metal magnetic component. Especially beneficial is the elimination of process steps previously required to form and stack a large number of individually punched laminations. Conventional punching dies are expensive to construct and have limited usable life when stamping amorphous metal. Moreover, the present process is more flexible in accommodating design changes without the detriment of having to amortize die fabrication costs over a large production run. Motors having large diameters can be made readily with efficient utilization of magnetic material that does not result in excessive production of unusable scrap. These benefits are difficult or impossible to achieve using conventional motors and the traditional production techniques associated therewith.
The electric machine of the invention is especially advantageous for applications requiring high efficiency, high rotational speed, and high power density. The reduced core losses afforded by the magnetic component of the invention increase the machine""s efficiency, the degree of improvement increasing with an increase in rotational speed. Moreover, the reduction in core losses over a component constructed using steels conventional in the motor art allows the present component and machine to be excited at higher frequency without incurring unacceptable heating due to the losses. The motor can therefore operate at higher rotational speeds. An increase in speed proportionally increases the power output for an equivalent torque level, leading also to a higher power density, i.e., a higher ratio of power output to motor weight.